An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

010RBISi A) to'1OToNS 1OF COMIfs. 265
are no1t full), agreed respecting the clements of its orbit, ol- its
periodic timet. Its elemllents resemble thlose of tic comllt of
1688, lwhich mwould give a period of:1.75 years; and to tfhis periodic tinlc, authlority at present inclines;X but -Prof. Ifltbbard,
of the Waslhington Observatory, after an elaborate discussion
of all the observations, tllinks the most probable period:170
Of all the comets onl recordl tlhe great comet of 181-:3 appl)oatchld nearest to thoe sun. It camel w\iitin a)out 0 G,0(00
nmiles of lis lumillous suirface, or only about one-fourth of the
distallce of theI moon fiom tlhe earth. lIt will be recollected
that to a spectator oil the ea rtll the sun's angular diameter blut
a little exceels ]half a deokree; l)tt were we to approachl as nelar
to the su;11 as thlis bo(y (lid in its perilhelion,' tlhat (iameter
would app)ear no less than 21.1 32'; tiand the light and lheat
(whichll increase as the square of the distance is (ilninished)
would be t47,i,00 times as grent as at present, the ]lcat exceeditn, nearlyt twenty-five titncs tllat prodtcced by Parkcrt's great
burning ll enlls, altllough thlis ilstrument is capab)le of prodnucing
effects beyond those of the most po-werful blast-furnace. Th'lhe
velocit~y of tho colnet was still mlore astonishing, being at the
ratc of more than one and a quarteor million of miles per' horll',
a x elocity sufficient to carry it throught 1800, or lhalf round the
sunll ill tw\ c hours.4|
Anl interestingl colmet appearedl ill 11858, called tlhe comet, of
)onaliti, who fi;st saw it at Florence, June 2d. It continlued in
silht till Oct. 15ttl. ITts tail, when\ l t perihelion, Oct. 10th,
was 60) long. llts nucleus was uncomm sonly bright, and be.
yonld it, ill the axis of the envelo)pe adlll tail was a dark, straight
line, like a shadow. Tllhe lonlg lcriod of its visibility gave unusual opportunity for careful observatioiis. TIts periodic timne is
variously estimated, froom 1,620 to 2,495 years.:
4-04. Of the physical'.aature of comets, little is understood.
It is usual to account fors the variations which their tails 1n-' Sco American Almanac for 1844, p. 91. Amer. Journal of Science, xlv.,
p. 188. AMstronomical Journal, vol. ii.,,. 1i6.
t Herschllel's Otlinest, p. 318.:' Sec 1ondll' "Account. of )olnatti's Co0met," with fine illustrations, in IMathl.
Mmthly, L)Dc., 1853.